C++ Void Pointers

Before you proceed with this tutorial, be sure to check C++ pointers.

Void Pointer

• A void pointer is A pointer that has no associated data type with it.
• A void pointer is a general purpose pointer that can hold address of any type and can be typecasted to any type.
• The void pointer doesn't mean it points to nothing. Actually, it is pointing something but we just don't know the exact type of object it is pointing to.

Syntax

void *ptr;

In C++, we cannot assign the address of a variable of one data type to a pointer of another data type.

Consider this example:

// pointer is of int type
int *ptr;

// variable is of double type
double d = 2.0;

// Error occurs
// we can't assign double* to int*
ptr = &d;

Here, the error occurred because the address is a double type variable. However, the pointer is of int type.

In such situations, we can use the void pointers (pointer to void) in C++.

For example:

// void pointer
void *ptr;
                                                                      
double d = 2.0;
                                                                      
// valid statement
ptr = &d;

We discuss earlier in this tutorial, The void pointer is a generic pointer that is used when we don't know the data type of the variable that the pointer points to.

Example 1: C++ Program how to use Void Pointer

#include <iostream>
using namespace std;
int main() {
    void* ptr;
    float f = 2.0f;

    // assign float address to void
    ptr = &f;

    cout << "The address of floating point no.: " << &f << endl;
    cout << "The address of void pointer no.: " << ptr << endl;

    return 0;
}

Output

The address of floating point no.: 0x7ffe1a5a540c
The address of void pointer no.: 0x7ffe1a5a540c

Here, the pointer ptr is given the value of &f.

The output shows that the void pointer ptr stores the address of a float variable f.

As void is an empty type, void pointers cannot be dereferenced.

void* ptr;
float* fptr;
float f = 2.0;
                                                                      
// assign float address to void pointer
ptr = &f;
cout << *ptr << endl;  // Error Occurs

// assign float address to float pointer
fptr = &f;
cout << *fptr << endl;   // Valid value

Example 2: Printing the Content of Void Pointer

To print the content of a void pointer, we use the static_cast operator. It converts the pointer from void* type to the respective data type of the address the pointer is storing:

#include <iostream>
using namespace std;
int main() {
    void* ptr;
    float f = 2.0f;
  
    // assign float address to void pointer
    ptr = &f;
  
    cout << "The Value of void pointer: ";
    // use type casting to print pointer content
    cout << *(static_cast <float*>(ptr));
  
    return 0;
  }

Output

The value of void pointer: 2.0

This program prints the value of the address pointed to by the void pointer ptr.

Since we cannot dereference a void pointer, we cannot use *ptr.

However, if we convert the void* pointer type to the float* type, we can use the value pointed to by the void pointer.

In this example, we have used the static_cast operator to convert the data type of the pointer from void* to float*.

Example 3: Print the Void Pointer using type casting

#include <iostream>
using namespace std;
int main() {
    void* ptr;// void pointer declaration
    int* ptr1;// integer pointer declaration
    int data = 2;// integer variable initialization
  
    ptr = &data; // storing the address of data variable in void pointer variable
    ptr1 = (int*)ptr; // assigning void pointer to integer pointer
    
    cout << "The Value of *ptr1: " << *ptr1 << endl;
  
    return 0;
  }

Output

The value of *ptr1: 2

Working

• In this above program, we declare two pointer variables of type void and int type respectively.
• We also create another interger type variable data.
• After declaration, we store the address of variable data in a void pointer variable ptr.
• Now, we want to assign the void pointer to integer pointer, in order to do this, we need to apply the cast ooperator ie,. int to the void pointer variable.
• This cast operator tells the compiler which type of value void pointer is holding.
• For casting, we have to type the data type and * in a bracket like (char) or (int).

void *ptr;
const double d = 2.0;

// Error occurs: Invalid conversion from const void* to void*
ptr = &d;

Next Tutorial

We hope that this tutorial helped you develop better understanding of the concept of Void Pointers in C++.

Keep Learning : )

In the next tutorial, you'll learn about C++ Pointer & Arrays.